1. Field of the Invention
The present invention relates to thermally assisted magnetic recording media having a high recording density. It also relates to thermally assisted magnetic recording systems including the thermally assisted magnetic recording medium, a probe of near-field light that irradiates the thermally assisted magnetic recording medium with light, and a magnetic read-write head.
2. Description of the Related Art
A magnetic disc system mounted typically on a computer is one of information storage systems that support present-day information society and is rapidly becoming higher in recording density, higher in speed, and smaller in size. In order for the magnetic disc system to achieve a high recording density, it is necessary, for example, to reduce the distance between a magnetic disc (magnetic recording medium) and a magnetic head, to reduce the size of crystal grains that form a magnetic recording layer of the magnetic recording medium, to increase the coercivity (or magnetic anisotropy field) of the magnetic recording medium, and to speed up signal processing.
In magnetic recording media, the diameter of magnetic grains existing in the magnetic recording layer is reduced as much as possible and crystal grain boundaries are provided between those magnetic grains to weaken the magnetic coupling between those magnetic grains, thereby reducing the noise of the recorded magnetization information. However, the resistance to the thermal energy goes lower if the volume of the magnetic grains is reduced, because the thermal energy required for maintaining the recording magnetization is in proportion to the volume of the respective magnetic grains.
One of possible solutions to the above problem is to increase the magnetic anisotropy energy Ku of the subject magnetic recording layer. The magnetic anisotropy energy Ku is a physical property value for representing the stability of magnetizing direction, that is, the difficulty of reversing the magnetizing direction. The magnetic anisotropy energy Ku is determined by the crystal structure and/or the material of magnetic grains. If the ambient temperature is defined as T, the volume of respective isolated magnetic grains is defined as V, and the Boltzmann constant is defined as kB, respectively, the magnetization reversal comes to occur more often under the influence of thermal fluctuation in reverse proportion to the value of KuV/kBT. Such frequency occurrence of thermal decay of magnetization can therefore be suppressed if the magnetic anisotropy energy Ku is increased to compensate for the reduction of the V value. A wide variety of materials having a high magnetic anisotropy energy Ku has been examined. Typically, a superlattice multilayer is now focused. The superlattice multilayer is a thin film formed by laminating two types of thin films alternately and artificially, each of the thin films having an atomic scale thickness and containing an element different from that of the other. The superlattice multilayer can thus have physical properties that do not exist naturally. As such superlattice multilayers, there are some well-known ones, each of which is obtained by laminating a ferromagnetic metal (Co, Fe) and a noble metal (Pd, Pt) alternately. Typically, Japanese Patent Application Laid-Open Publication No. H05(1993)-67322 discloses a perpendicularly magnetized layer that uses a Co/Pt superlattice multilayer. Japanese Patent Application Laid-Open Publication No. 2003-141719 and Japanese Patent Application Laid-Open Publication No. H06(1994)-111403 respectively disclose perpendicularly magnetized layers using a superlattice multilayer. All those media are so-called “continuous layers” having no grain boundary between grains. Media using an alloy of a ferromagnetic metal (Co, Fe) and a noble metal (Pd, Pt) as a magnetic recording layer, such as media disclosed in Japanese Patent Application Laid-Open Publication No. 2000-67425 and Japanese Patent Application Laid-Open Publication No. 2008-84413, also have a high magnetic anisotropy energy Ku. These media, however, are also continuous layers.
A technique for promoting segregation of magnetic grains from one another has been employed in known media for use in hard disc drives (HDDs). The technique forms grain boundaries by adding oxide to the subject magnetic metal layer. Typically, if a magnetic metal alloy such as CoCrPt (cobalt-chromium-platinum alloy) and a non-metal material such as SiO2 are vacuum-deposited at the same time under predetermined conditions, mesh-like oxide grain boundaries are formed so as to surround the magnetic metal alloy grains, respectively. Media manufactured by this technique are generally referred to as granular media (Appl. Phys. Lett. 52(1988) p. 512). The noise of the granular media is extremely reduced, because the magnetic grains are segregated from one another by a non-magnetic oxide layer, the magnetic exchange-coupling therebetween is thereby weak and the magnetic crystal grains are formed finely.
Under these circumstances, Japanese Patent Application Laid-Open Publication No. 2002-25032 and Japanese Patent Application Laid-Open Publication No. 2005-190538 each disclose a technique for converting a superlattice multilayer having a high magnetic anisotropy energy Ku into a granular medium. The resulting granulated superlattice multilayer is supposed to achieve a high recording density.
Increase in anisotropy energy, namely, increase in magnetic anisotropy field (or coercivity) also means increase in head-field intensity required for recording. According to current techniques, however, the head-field intensities are hitting a ceiling. The ceiling is made typically by the material of magnetic pole used in write head, and the distance between a magnetic disc and a magnetic head. In other words, it becomes difficult to further reduce the distance between the magnetic disc and the magnetic head. Under these conditions, it becomes difficult to carry out recording even on media using CoCr alloys, and it is more difficult to carry out recording on superlattice multilayers and on alloy layers made of anisotropic materials. As a possible solution to this problem, a variety of media using CoCr alloys and having two or more magnetic layers has been proposed. As exemplary media having two magnetic layers, exchange coupling media (ECC media) have been proposed typically in US Patent Application Publication No. 2007/0212574. In the exchange coupling media, one of the two magnetic layers is a soft-magnetic layer that is susceptible to magnetization reversal (magnetization rotation) or a continuous layer analogous thereto, and the other magnetic layer is a granular layer made from a hard magnetic material. The soft-magnetic layer or continuous layer undergoes magnetization reversal at a bottom magnetic field and thereby undergoes switching prior to the hard-magnetic layer (hereinafter the former is referred to as a “switching layer”). The switching layer, once undergoing magnetization reversal, accelerates the magnetization reversal of the magnetic recording layer (hard-magnetic layer) to carry out recording, because the upper and lower magnetic layers magnetically interact with each other.
Hybrid recording techniques using both an optical recording technique and a magnetic recoding technique are supposed to be effective for solving the ceiling problem in head-field intensity. The techniques are effective typically when a higher magnetic anisotropy energy Ku is required for proving a higher recoding density, as in superlattice multilayers and highly anisotropic alloy layers. Typically, Jpn. J. Appl. Phys. 38(1999), p. 1839 discloses a technique using a specific read-write head. In the read-write head, a mechanism for light generation is added to a portion where a recording magnetic field is generated. According to this technique, light is generated in addition to an applied magnetic field upon recording, and the recording is performed on a medium while heating the medium to reduce the anisotropy energy (magnetic anisotropy field or coercivity). This technique enables easy recording even on media having a high coercivity for use in recording at ultra-high density. In contrast, recording on such media having a high coercivity is difficult by using common magnetic heads because of insufficient recording magnetic field. However, a heat generating device has to rapidly heat and cool a tiny heating region to achieve a high recording density for such magnetic disc system. Accordingly, there is a limit to the approach of focusing laser light through a lens generally used for optical recording. The approach of generating near-field light by a metallic surface plasmon is proposed as a possible solution for solving this, and studies are carried out (Optics Japan 2002 Extended Abstracts, 3pA6 (2002); and Japanese Patent Application Laid-Open Publication No. 2003-45004). The reading herein uses a giant magnetic resistance (GMR) head or tunneling magnetoresistive (TMR) head as in common magnetic recording. This recording technique is referred to as thermally assisted magnetic recording.
Recording techniques close to the thermally assisted magnetic recording technique include an optical magnetic recording technique (“Hikari To Jiki (in Japanese; Light and Magnetism)”, Katsuaki SATO, Asakura Publishing Co., Ltd., Tokyo Japan, (2001), p. 156). The optical magnetic recording technique utilizes, for magnetic recording, a change in magnetic properties based on a temperature increase in the medium by laser light irradiation. Such optical magnetic recording techniques are classified as several types by the recording procedure. One of them is a recording technique that involves heating, to Curie temperature, a medium typically using TbFe (terbium-iron alloy) or GdTbFe (gadolinium-terbium-iron alloy). Specifically, spontaneous magnetization decreases sharply in the vicinity of the Curie temperature, and paramagnetism develops at or above the Curie temperature. At this time, a magnetic field is applied in an opposite direction. In a cooling process, magnetization reversal occurs, so that a mark is recorded. Another type is a recording technique that involves heating a medium made of GdFeO (gadolinium-iron-oxygen alloy) or GdCo (gadolinium-cobalt alloy) at or above a compensation temperature. This technique utilizes a phenomenon given below. When two sublattice magnetizations of ferrimagnetism compensate each other at a temperature on which these magnetizations are dependent, macroscopic magnetization becomes zero (which is called a “compensation temperature”), so that coercivity is maximized. When such a material as has a compensation temperature at room temperature is heated at or above the compensation temperature, therefore, the coercivity is reduced, so that magnetization is oriented in the direction of an external magnetic field. According to current techniques, however, the recording technique relating to the Curie temperature and the recoding technique relating to the compensation temperature are used in combination. In any case, the medium is made of an amorphous alloy film of rare earth and transition metal. A record mark is determined by forming a cylindrical magnetic domain. Formation of the magnetic domain is determined by a balance between some forms of magnetic energy (such as external magnetization energy and magnetic domain wall energy) acting on the medium. Because of having no grain boundary, the amorphous alloy film has the merit of achieving a low noise level as compared to a CoCr-based granular medium that has been used for magnetic discs. However, as the spot size of light becomes smaller, the record mark may possibly become larger than the spot size of light or become rather smaller and disappear. Consequently, the amorphous alloy film is considered to be unsuitable for high recording densities.
The thermally assisted magnetic recording is characterized by facilitating recording by heating the medium to reduce the magnetic anisotropy field intensity (or the coercivity) of the medium, as mentioned above. In other words, the magnetic anisotropy field intensity of the medium has dependence on temperature, and therefore the magnetic anisotropy field intensity becomes lower as the temperature of the medium becomes higher (J. Appl. Phys. 91, 10(2002) p. 6595). In the thermally assisted magnetic recording, the medium should be heated to its Curie temperature or higher (see The 2008 IEEE International Magnetics Conference (Intermag 2008) Digest AE-05 and Intermag 2008 Digest AE-07). Typically, by taking a medium made of a FeNiPt alloy as an example, its Curie temperature elevates with an increasing magnetic anisotropy field Hk and is about 750 K at a magnetic anisotropy field Hk of 80 kOe (J. Appl. Phys. 91, 10(2002) p. 6595). However, it is desired to carry out heating at a temperature as low as possible, because the medium, if heated to such a high temperature, can plastically deform and thereby have deteriorated magnetic properties.
Accordingly, it is important to examine materials and structures of media for carrying out thermally assisted magnetic recording at a temperature as low as possible. Typically, Japanese Patent Application Laid-Open Publication No. 2000-293802, Japanese Patent Application Laid-Open Publication No. 2001-76331, Japanese Patent Application Laid-Open Publication No. 2002-358616, Japanese Patent Application Laid-Open Publication No. 2008-52869, and US Patent Application Publication No. 2002/0192506 disclose techniques in which a medium including two or more magnetic recording layers is used and recording thereon is performed at a temperature that is higher than the Curie temperature of one of the two layers (assuming for example a lower layer) but is equal to or lower than the Curie temperature of the other layer (assuming for example an upper layer). These techniques perform recoding according to a mechanism as follows. Specifically, the magnetization of the lower layer whose Curie temperature is below the recording temperature is minimized or disappear to decouple the magnetic exchange coupling between the lower and upper layers. Then recording is performed on the upper layer whose Curie temperature is higher than the recording temperature (whose magnetization remains), and the magnetic exchange coupling again occurs during cooling process of the medium to thereby transfer the magnetization of the upper layer to the lower layer. Thus, recording is performed. The upper and lower layers refer to as a recording layer and a transfer layer, respectively.